Single model site = primarily focused on a single model usually a porn star.

Solo site = many different guys jacking off usually alone.

Everytime "Solo Model Sites" is brought up, there is confusion on what it means. Why can't we agree to call it something different?

And to answer the question, hardcore seems to be an easier sell. And if a surfer is fixated on a porn star/model they'll join regardless if it's solo or hardcore. But if you throw in some hardcore action, you'll attract other surfers that may not be that fond of the main model.